1. Field of the Invention
The invention relates generally to autoimmunity and specifically to a novel protein which contains an autoepitope and methods of detecting autoantibodies that bind to the autoepitope.
2. Description of Related Art
Binding of autoantibody to the senescent cell membrane is a major physiologic pathway for removal of senescent cells and is important in many disease states, such as sickle cell anemia and thalassemia (Turrini, et al., J. Biol. Chem., 266:23611-23617, 1991). It has been determined that the chronology of RBC senescence is first, clustering of the integral membrane proteins (AE1) and glycophorin A (GPA), followed by autologous IgG binding, complement deposition, and phagocytosis. Clustering of AE1 and GPA within the RBC membrane leads to the close approximation of bound IgG molecules, resulting in complement activation, and deposition of complement components on the RBC surface. Subsequently, RBCs bind to phagocyte Fc and C3b receptors, and are engulfed and destroyed.
It has been shown that synthetic peptides corresponding to two autoepitopes of AE1, NSSYFPGKL (SEQ ID NO:4) and FKPPKY (SEQ ID NO:5), inhibit binding of IgG eluted from aged RBCs (Kay, M. M., et al., Proc. Natl. Acad. Sci. U.S.A., 87:5734-5738, 1990). These peptides, when used together, produce greater inhibition of IgG binding to aged RBCs than either peptide used alone. However, since AE1 is present in all RBCs, young and old alike, some change in AE1 must occur to cause expression of an autoantigen that binds IgG. Obviously, identification and characterization the antoantigens which bind to IgG are important in understanding the mechanisms of phagocyte-mediated removal of senescent cells.
Identification of autoantigens is also essential to understanding and to intervention in the autoimmune response. For example, in autoimmune hemolytic anemia (AHA), autoantibody binding to autoantigen causes the clinical disorder, hemolysis. Yet such patients vary considerably with respect to this degree of hemolysis and response to treatment. Some individuals have IgG-coated RBCs (positive DAT) without hemolysis. Although some differences can be explained by variability in the autoantibody, such as IgG subclass, most differences are unexplained and must relate to the autoantigen detected.
Furthermore, autoantibodies are also found in other autoimmune disorders, in malignancies, in infectious diseases such as the acquired immunodeficiency syndrome, as well as in apparently healthy individuals. Clinically-relevant autoantigens have been cloned in several autoimmune disorders. For example, clones expressing the La autoantigen of Sjogren's syndrome (Chambers, et al., J. Biol. Chem., 263:18043-18051, 1988) the M2 mitochondrial autoantigen of primary biliary cirrhosis (Coppel, et al., Proc. Natl. Acad. Sci., U.S.A., 85:7317-7321, 1988) the Sm and p70 (KU) autoantigens of systemic lupus erythematosis (Ohosone, et al., Proc. Natl. Acad. Sci, U.S.A., 86:4249-4253, 1989; Reeves, et al., J. Biol. Chem., 264:5047-5052, 1989).
At present, complex native autoantigens such as RBC membrane proteins are used to detect and characterize RBC autoantibodies. However, since only limited regions of molecules appear to be involved in an immune response, central to understanding the immune response is understanding the nature of antigenic determinants that stimulate the immune response.
Therefore, identification of autoantigens and the polynucleotides encoding them is important in understanding the mechanisms involved in the removal of senescent cells and the autoimmune response. The identity of such autoantigens is also important in development of diagnostics and therapeutics for intervention or inhibition of an undesired immune response. The present invention identifies a novel autoantigen, PINCH, that binds IgG specific, inter alia, for aged cells. Thus, the PINCH autoantigen is useful for the in vitro and in vivo removal from susceptible individuals of autoantibodies which bind aged RBCs and other cells, thereby increasing the lifespan of such cells and, at least for aged RBCs, reducing the need for transfusion.